It was Onyekuru’s first game for Galatasaray since return to the club on loan from AS Monaco.
Onyekuru provided the assist for Algerian star Sofiane Feghouli who scored Galatasaray’s fourth goal in the 94th minute.
The goal was Feghouli’s second after netting the third goal in the 64th minute.
Adem Buyuk opened scoring for Galatasaray in the 5th minute before Ryan Donk made it 2-0 in the 21st minute.
Kayserispor goal was scored by Muris Mesanovic on 69 minutes.
It was a fourth straight win for Galatasaray who are now sixth on 36 points, five points away from leaders Sivasspor in the league table.